Are you talking about storm windows, or replacing the entire window?
If you're talking about replacement windows, find a contractor that uses good windows, like Simonton or Certainteed. I've heard that Thompson Creek windows are pretty good, too. If you can't do a web search on the window brand, think twice.
The Anderson replacement windows (Renewal) are beautiful, but at quite a premium.
I installed Simonton windows (myself). Replacement windows make an enormous difference, but it takes quite a while for the investement to pay off.
Installing a standard double hung replacement window, including cladding, calking, and insulating around the window could be $500 a window. But depending on how much work the contractor has, it could be significantly cheaper, so shop around.
THINKING OF REPLACING STORM WINDOWS WITH MORE EFFICIENT ONES; ESPECIALLY THE ONES THAT TILT FOR EASY CLEANING. ANYONE HAS HAD EXPERIENCE RECENTLY WITH THAT?
PRICE RANGES?